The transparent conductive films have a high conductivity (for example, a resistivity of 1×10−3 Ωcm or less) and a high transmittance in the visible region and thus are utilized as the electrodes of solar cells, liquid crystal display elements and other various light-receiving elements, followed by heat wave reflective films used for window glass of cars and buildings, various antistatic films, and antifogging transparent heating elements for refrigerating show cases.
As the transparent conductive films, antimony- or fluorine-doped tin oxide (SnO2) films, aluminum- or gallium-doped zinc oxide (ZnO) films, and tin-doped indium oxide (In2O3) films are widely utilized. In particular, the tin-doped indium oxide films, namely In2O3—Sn based films, are referred to as ITO (indium tin oxide) films and are widely used in various devices, not to speak of LCDs, because low-resistance transparent conductive films are easily obtained.
The ITO film is also used as the electrode of the organic EL element. Since the organic EL element is such that driving voltage is considerably lowered and thereby its compact design is facilitated, researches concerning the practical use of the organic EL element as a display element of the next generation are actively conducted. The organic EL element is designed so that the lamination of an anode/a light-emitting layer/a cathode is fundamentally used and generally, the ITO film is formed as a transparent anode on a substrate using a glass plate or the like. Positive holes are injected from the anode and thus it is desirable that the work function of the anode is rather high. In this case, a work function of at least 4.4 eV is required, but it is desirable to be 5.0 eV or more. However, it is generally known that the work function of the ITO film is about 4.4 eV, and this work function is not sufficient to use the ITO film as the anode of the organic EL element.
Generally, as set forth in Non-Patent References 1 described below, it is necessary to give the surface of the ITO film acid cleaning and UV ozone cleaning and to increase the work function to approximately 5.0 eV. However, the work function is highly sensitive to a processing condition and thus its control is difficult. It is also reported that the work function is sometimes returned to a value before processing by leaving the ITO film for a certain time.
Patent Reference 1 described below proposes the organic electroluminescence element in which an organic layer containing an organic light-emitting layer is sandwiched between the anode and the cathode so that the cathode comprises, in order from a side coming in contact with the organic layer, an electron injection electrode layer, a transparent conductive film, and a metallic thin film with a resistivity of 1×10−5 Ωcm or less, laminated one over another and a transparent thin film layer is formed outside the cathode. As this transparent thin film layer, an amorphous transparent conductive film using an oxide composed of indium (In), zinc (Zn), and oxygen (O), namely a so-called IZO film, is used.
The IZO film is a stable amorphous film in which not only is the resistivity low, but also microcrystals are not contained, and hence is useful for the electrode film of the organic EL element. However, the work function of the IZO film is about 4.8-4.9 eV, which is not said to be high enough to use the IZO film as the anode of the organic EL element. It becomes necessary to improve the work function by surface treatment as in the ITO film.
As mentioned above, the transparent conductive film which is useful for the anode of the organic EL element, high in work function, and high in conductivity has not yet been produced and its development is expected.
Patent Reference 1: Japanese Patent Kokai No. Hei 7-182924
Non-Patent Reference 1: “New development of transparent conductive film II”, CMC Press, First edition, p. 203.